Established Standards promulgated by code making authorities relating to devices or modules commercially known as receptacle type Ground Fault Circuit Interrupters and hereinafter designated as GFCIs that are installed in wall receptacles require the GFCI to be a two-pole device. This requirement exists to assure that a ground fault between a line conductor and ground will be cleared in event the GFCI is miswired, i.e., the line and neutral conductors are reversed at the input terminals of the device. Compliance with the standard is commonly achieved by the use of mechanically latched, magnetically operated mechanisms in GFCI receptacles. This type of mechanism requires power to unlatch and trip the devices because GFCI are commonly permanently installed and wiring continuity can thus be assured.
Safety considerations require that GFCI mechanisms be non-teasable or trip free as a situation can be postulated in which the GFCI mechanism is held in a position where the hot line is conductive while the neutral line is open. This type of condition is transient as the condition must be maintained by an external force. In mechanisms of this type if the electronics and the disconnect means are powered downstream from the trip mechanism, a ground fault on the load side of the GFCI may be present while the GFCI is without power to interrupt the circuit in which it is installed.
Two methods are presently used to overcome the problem. Both methods are effective but include certain disadvantages.
Coil Clearing Contacts: Coil clearing contacts allow the electronics and disconnect mechanism in the GFCI to be powered from the line side of the power contacts. This assures that the GFCI will have power to trip even though the neutral contact may be teased open. The coil clearing contact is synchronized so that it opens after both line contacts open and closes before the line contacts close. This assures that the GFCI is always supplied with power during any situation where a ground fault may be present. It follows that it is necessary that the power from the GFCI be removed after the GFCI is tripped as the steady state current required to trip the GFCI, if uncleared, would damage the GFCI. A major disadvantage of coil clearing contacts is the mechanical complexity required. Coil clearing contacts also decrease the overall reliability of the GFCI. Coil clearing contacts are essentially dry contacts as they conduct only a few milliamperes of power except during the brief interval the GFCI is tripped.
Electronic Commutation: This method is similar to the coil clearing contacts except that the tripping mechanism, normally a thyristor, is connected in the half-wave mode to the line side of the GFCI disconnect contacts. Once a fault is detected, the thyristor is turned on, applies power to the disconnect mechanism, and clears the fault. When the power is disconnected from the fault, the SCR commutates off. Since the fault is no longer present, the thyristor does not subsequently become conductive. The disadvantage of this method is that a failure of the SCR may cause excessive damaging currents in the solenoid coil.